Exhaust Gas Recirculation has been utilized for a number of years in both compression ignition and spark ignition engines as a means of reducing exhaust emissions. As is well known, nitrogen oxides, primarily nitric oxide (NO) and nitrogen dioxide (NO2) (collectively referred to as “NOx”) are undesirable byproducts of combustion, which are formed when the nitrogen in air is exposed to high temperatures, such as in an internal combustion engine. As is also well-known, exhaust gas recirculation is an effective means of reducing the combustion temperature of an internal combustion engine. Exhaust gas re-circulation involves introducing a metered amount of exhaust gas into the cylinder during the intake stroke along with the fresh charge of air/fuel mixture. The exhaust gas displaces a portion of the air/fuel mixture entering the cylinder which reduces the combustion temperature thereby inhibiting the formation of NOx.
An EGR cooler is a component in the exhaust gas recirculation system that cools the exhaust gas before it is re-introduced into the combustion cylinder. Cooling the exhaust gas enables a cooler, denser charge of re-circulated exhaust gas to be introduced into the combustion cylinder, thereby further reducing combustion temperatures and NOx emissions.
The EGR cooler itself is a heat exchanger that utilizes engine coolant to remove heat from the exhaust gas flowing through it. The heat exchanger design keeps the coolant and exhaust gas from mixing, but allows for heat to flow from the exhaust gas to the engine coolant. Typically, the EGR cooler is a tube-and-shell heat exchanger in which a shell structure surrounds a plurality of tubes that run through the shell. Exhaust gas is passed through the tubes while engine coolant is circulated through the shell. Heat from the exhaust gas is transferred to the coolant in contact with the tubes. The coolant then passes through the water jacket of the engine and finally to the radiator where it is cooled by air passing through the radiator fins. In summary, the EGR cooler is a heat exchanger through which engine coolant can circulate to extract heat from a portion of the spent exhaust gases which are cooled and redirected back to the engine intake manifold such to reduce the creation of NOx vehicle emissions.
Original Equipment Manufacturer (“OEM”) EGR coolers can fail for various reasons. Their failure may range from degradation of performance with concomitant increase in NOx emissions to a catastrophic failure that disables the vehicle. A common catastrophic failure is caused by a plugged oil cooler depriving the EGR cooler of coolant. If coolant flow to the EGR cooler is reduced, the EGR cooler can be subjected to excessively high exhaust gas temperatures. The resulting extreme temperature rise in the EGR cooling unit can rupture or crack the internal pathways of the EGR cooler. Ruptures can occur due to reduced coolant flow causing temperatures above the boiling point of the coolant. The coolant may “flash boil” in the EGR cooler, especially if circulation is reduced allowing the coolant to remain on the hot spot long enough to boil. Boiling can cause a rupture within the ECR cooler when the liquid coolant is suddenly turned into an expanding gas. This rupturing can lead to the mixing of the exhaust and engine coolant through the in ruptures or cracks in the EGR cooler. This type of catastrophic malfunction can cascade into the other engine systems, damaging the engine and its associated components.
Another failure causing reduced effectiveness of the EGR cooler, and degraded EGR system performance can arise due to the internal configuration of the conventional EGR cooler. In conventional OEM EGR cooler designs only a few tubes (typically oval in shape) are incorporated in the construction. The tubes may contain baffling, or thin heat exchange fins on the interior of the tubes to increase the surface area available for heat exchange. This finned structure, however, can lead to another failure mechanism through clogging of the exhaust gas passageways. The conventional tube and fin heat exchange EGR cooler designs can become clogged over time due to a buildup of particulates that can be found in the exhaust and other factors, resulting in substantially reduced heat transfer capability and higher exhaust gas temperatures in the intake manifold. The higher temperature re-circulated exhaust gas leads to increased NOx production due to higher combustion temperatures. Accordingly, there exists a need for an improved EGR cooler that is less prone to catastrophic failure and performance degradation, including clogging in the exhaust passage ways, rupturing when coolant flow is reduced, and other failure, or performance reducing mechanisms. It is also desirable to provide a method of rebuilding salvaged conventional EGR coolers to solve the abovementioned and other problems.